Post-translational modification of endogenous proteins by the introduction of phosphate groups occurs in peripheral cardiovascular tissues and in cardiovascular centers of the brain stem. The reaction is calyzed by protein kinase, which can be activated by cAMP and cGMP, both of which are thought to be involved in blood pressure control. Protein kinase was characterized kinetically in brain stem, aorta and heart, and saturating conditions were established for the determination of Vmax. During the development of hypertension of spontaneous origin, there was no apparent difference in cyclic nucleotide-independent and cyclic nucleotide-dependent protein kinase activity in brain stem, aorta or heart of the spontaneously hypertensive rat (SHR) when compared to age-matched normotensive Wistar-Kyoto (NT-WKY) controls. However, significant age-related decreases in total kinase activity were observed in the aortas of SHR. These decreases were not observed in any other tissue studied. The significance of the drop in aortic kinase activity in SHR in relationship to blood pressure control is not understood at this time. Our laboratory is presently engaged in studying changes in endogenous substrate availability for protein kinase in the above tissues of the SHR.